OMAHA, Neb. — After firing his fourth pitch of the third inning of his start against Oregon State, LSU freshman right-hander Eric Walker stepped off the mound and looked toward the dugout in apparent discomfort. 

It was tightness in his forearm, a sensation that flared up for the first time last week during a simulated game. After visiting with LSU coach Paul Mainieri, pitching coach Alan Dunn and athletic trainer Cory Couture, Walker exited the game. 

Mainieri confirmed the forearm tightness during an in-game interview with ESPN, saying it first started bothering Walker as he was warming up before the third inning. 

"In his third warmup pitch before the third inning he just felt his forearm tighten up," Mainieri said. "It became unbearable so we had to take him out." 

Mainieri added that he didn't think anything was "structurally wrong. He just had a fatigued muscle." 

Sophomore right-hander Caleb Gilbert replaced Walker on the mound. 

In two innings, Walker allowed two hits and one run. He had thrown 24 pitches before his exit. 

Mainieri disclosed Walker's initial forearm tightness during an interview session Friday. LSU worked Walker out Saturday morning before its first College World Series game at nearby Creighton and determined him to be fine to pitch. 

"His bullpen was great (Saturday), he had no ill effects after, he had no ill effects (Sunday) morning." Mainieri said at LSU's practice Sunday afternooon. 

Mainieri was neither asked if Walker would pitch again in the CWS, nor did he speculate on his availability moving forward. 

Walker (8-2) was charged with the loss, his first since he went 2.1 innings April 23 at Kentucky in what was before Monday his shortest outing of the season.

Follow Luke Johnson on Twitter, @ByLukeJohnson.